Experimental study on the effects of using metal foam on R-134a flow boiling in annular tubes

Heat transfer coefficient and pressure drop of the refrigerant R134a flow boiling in metal-foam filled annular tubes are studied. Three test tubes are analyzed. One tube is plain annular and the others are filled with 10 and 20 PPI copper foam pieces. The three test tubes are dimensionally similar with a length of 30 cm, outer and inner diameters of 14.3 mm and 4.7 mm, respectively. The experiments are performed for mass flux ranging from 10 to 80 kgm(-2)s(-1) and vapor quality of 0.13-0.85. Also, a constant and uniform heat flux of 5.7 kWm(-2) is applied to the test tube in all of the experiments. It is shown that boiling heat transfer coefficient and pressure drop of the refrigerant profoundly increases using metal foam. Furthermore, they are increased by an increment in pore density, and in particular, heat transfer coefficient is enhanced up to 220% using 20 PPI metal foam. Evidently, both heat transfer and pressure drop are proportionally varying with mass flux. Although these parameters generally increase with an increment in vapor quality, heat transfer coefficient decrement is observed at high vapor quality and low mass flux values. Based on the test data, and due to the lack of existing correlation to span over the testing conditions in this experiment, two new correlations are proposed. New correlations can predict boiling heat transfer coefficient and two-phase pressure drop of refrigerant in the metal-foam filled annular tubes with good accuracies.